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Coupling between nonlinear elasticity and stress relaxation in biopolymer gels

The extracellular matrix (ECM) is a soft matrix of structural proteins that provides physical support and biochemical signaling to cells in tissues. The mechanical properties of the ECM have been found to play a key role in regulating various biological processes including stem cell differentiation, and malignancy. Gels formed from ECM protein biopolymers such as type Ι collagen or fibrin are commonly used to mimic the ECM microenvironment for 3D cell culture models of tissue. Interestingly, these gels exhibit nonlinear elasticity and undergo strain stiffening at low strains. However, these gels are also viscoelastic and exhibit stress relaxation, with the resistance of the gel to a deformation relaxing over time. Here we examine the coupling between nonlinear elasticity and viscoelasticity in biological gels. We measure the strain dependent stress relaxation of biopolymer gels and find that at higher strains, not only do biopolymer gels stiffen, but they also exhibit faster stress relaxation, reducing the timescale over which elastic energy is dissipated. This effect is not universal to all biological gels, and is mitigated through covalent crosslinks. A computational model of biopolymer networks, which included force dependent unbinding of biopolymers from a network under strain followed by slow rebinding of the biopolymers, quantitatively captures the experimentally measured results. These results highlight the interplay between nonlinear elasticity and viscoelasticity in biopolymer gels.

Author(s):

Sungmin Nam    
Stanford University
United States

Ovijit Chaudhuri    
Stanford University
United States